Ultrasonic transducers, and in particular, phased array ultrasonic transducers, are frequently utilized for a variety of medical applications. In one such application, the transducer is disposed at the end of an endoscope which is suitably positioned in the patient's esophagus for scanning such internal organs as the heart. When used in this manner, this transducer or probe is referred to as a transesophageal probe and the procedure is referred to as transesophageal echocardiography (TEE) when the probe is utilized for scanning the heart. Other invasive probes which have similar structures and requirements include transrectal, transnasal and transvaginal probes. The shaft of the endoscope serves as an enclosure for electrical and mechanical cables which couple the transducer and other electrical components of the endoscope to an external power source and external controls. Since a probe of this type and its shaft is positioned inside the body, the probe and shaft must be sealed to protect them against attack from bodily fluids and acids, as well as against sterilizing solutions and cleaning solutions either inside or outside the body. Moreover, the probe and shaft must protect the patient from currents carried by the electrical cables. Thus, the transducer as well as the shaft of the endoscope must be enclosed within an insulative, protective outer covering. Such a covering also helps protect the body from irritation as a result of probe rotation.
Existing shafts of such transesophageal probes typically comprise an inner, convoluted metal core which provides the required crush resistance to the shaft; a stainless steel braided sheath which is constructed over this inner core; and a outer coating of an elastomeric material. The elastomeric material serves as an insulator to protect the patient from electric currents; a smooth, corrosion-resistant surface to facilitate the placement of the probe; and a cover to protect the mechanical and electrical components of the endoscope from damage by bodily fluids.
One of the major causes of failure of this type of probe shaft is the physical penetration of this outer elastomeric coating by the patient. Penetration can result from the teeth of the patient being clamped tightly about the shaft, or from the shaft being rubbed against the patient's teeth during insertion of the probe. Even with the most cut-through resistant, flexible coatings available, such as urethanes, eventually the coating is penetrated by the patient's teeth, or by wear and tear from other sources. These penetrations are exacerbated by exposure to stomach acid, cold sterilants and cleaning fluids commonly used in a clinical setting. Once the elastomeric coating is penetrated, there is a direct, electrically conductive path between the patient and the probe, thereby introducing a potential safety risk to the patient. Even a single small penetration of the outer elastomeric coating can provide a direct electrical path to the patient, once the probe has been inserted into the esophagus.
Another important consideration is proper and complete sterilization of the probe and probe shaft. Cuts in the elastomeric coating can allow bodily fluids or bacteria to become trapped therein, particularly if the probe shaft is bent in such a way as to press together the edges of the cut after entry of fluids or bacteria. In such instances, the sterilization process might not completely flush out such bodily fluids and destroy the bacteria and subsequent patients could be contaminated. Also, if the outer elastomeric coating is completely pierced, bodily fluids and bacteria could enter the space between the braided sheath and the outer coating and travel longitudinally along the length of the probe shaft. Such fluids and bacteria would become trapped and could survive conventional sterilization procedures.
In designing shafts for invasive bodily probes, particularly for transesophageal probes, it is important that the shaft diameter be as small as possible. This shaft is the major portion of the probe that occupies the esophagus of the patient during the examination, and larger diameter shafts create tremendous patient discomfort during insertion and use. Any solution to existing problems of sterilization and penetration of the outer elastomeric coating by the patient should not significantly increase the outer diameter of the shaft. Thus, merely making the outer covering thicker is not a viable solution to these problems. Also, a thicker outer covering does not significantly reduce the likelihood of cut-throughs. It is also important not to significantly alter the inner diameter of the shaft, or the diameter of the metal core. The core diameter defines the space available for containment of the electrical and mechanical cables, and the demands of the probe place severe restrictions on the amount to which the diameter of the core can be reduced.
An improvement on existing probe shafts and one solution to the foregoing problems is found in U.S. Pat. No. 5,156,155, issued in the name of the applicant herein and assigned to the assignee of the present application. In U.S. Pat. No. 5,156,155, the conventional transesophageal probe shaft is modified to include a dielectric sleeve which is disposed between the core and sheath for electrically isolating the core from the sheath. A dielectric spacer is also provided in conjunction with a fitting disposed at the end of the shaft which completely seals the core from other portions of the probe to protect the patient from electrical currents carried by wires within the core and to protect the wires from damage from bodily fluids and cleaning solutions.